Electric motors are used almost everywhere in society to produce different mechanical movements, for example, rotating pumps and fans. There are numerous different types of electric motors, of which the most common is the so-called squirrel-cage motor.
The connection of a squirrel-cage motor to the network is known to cause a substantial switching current surge; the current taken from the network when starting can transiently be over 6 times the rated current. This kind of current surge often causes problems, such as the need to dimension the fuses and cables of the supply circuit to be larger than the load during actual operation would require, as well as the extra costs incurred by this kind of over-dimensioning. Generally the larger the power output of the motor is in question, the larger problem the switching current surge is.
One prior-art solution for reducing the starting current is to use a so-called soft starter, which may include a circuit implemented with thyristors, with its control unit, and in which the control angle of the thyristors is controlled so that the voltage of the motor decreases to avoid the over large current of the starting phase. This type of solution is known, for example from publications DE4406794 and U.S. Pat. No. 5,859,514. A drawback of the solution is the cost of starting and the power loss during operation as the motor current runs continuously through it. In order to reduce continuous power loss, the prior art solution bypass the soft starter by connecting the motor directly to the network after the starting phase with a shunt contactor.
The use of a frequency converter for starting the motor without a switching current surge is also a well-known solution. When the load requires a varying speed of rotation, the use of a frequency converter is otherwise a natural solution. If, however, the load of the motor allows continuous operation at a fixed frequency of the supply network, the prior art solutions use a shunt circuit implemented with contactors to minimize power losses, with which shunt circuit the motor is disconnected after the starting phase from the frequency converter and connected directly to the network. A shunt circuit may be used in the prior-art pump automatics according to FIG. 2a, in which one frequency converter and a number of motors of which each can be connected either to the frequency converter, or directly to the network. With the solution the total flow produced by the pumps can be steplessly adjusted from zero up to maximum delivery, in which case all the motors operate at their rated speed.
Also a motor accelerated to its rated speed with a frequency converter can take a substantial connection current surge, even greater than the starting situation, when it is connected directly to the network. This occurs if the amplitude and phase angle of the so-called residual voltage evident in the connectors of the motor after disconnecting from the frequency converter differ from the amplitude and phase angle of the supply network at the time when the motor is connected directly to the network. Owing to the deceleration of a loaded motor and the switching delays of the contactors, which can be in the range 40 . . . 100 ms, a simple and reasonably priced prior-art method or arrangement to synchronize the residual voltage and the voltage of the supply network at the time of connection is not found.